The present invention relates to a metallic powder-molded body, a re-compacted body of the molded body and a sintered body produced from the re-compacted body, which are suitable for the manufacture of various structural machine parts made of sintered metals, and processes for the production thereof.
The process for making sintered metals essentially includes mixing of powder as a raw material, compaction, sintering and after-treatment (heat treatment). Although the sintered products can be produced only through these essential steps, in many cases, additional steps or various treatments are performed between or after the essential steps according to requirements.
For instance, Japanese Patent Application First Publication No. 1-123005 discloses a process comprising the steps of compacting a mixed powder to form a preform, provisionally sintering the preform to form a metallic powder-molded body, re-compacting (cold forging) the metallic powder-molded body and then sintering (substantial sintering) the re-compacted body.
Specifically, in the conventional process, the re-compaction (cold forging) step of the metallic powder-molded body is constituted by a provisional compaction step and a substantial compaction step. The metallic powder-molded body is provisionally compacted after applying a liquid lubricant to a surface thereof, and exposed to negative-pressure to absorb and remove the lubricant therefrom. Then, the metallic powder-molded body is subjected to substantial compaction step.
Since these steps allow the lubricant to still remain in an interior of the preform, micropores within the preform can be prevented from being collapsed and eliminated, thereby inhibiting the preform from suffering from a porous structure. As a result, the density of the obtained product increases up to 7.4-7.5 g/cm3, thereby enabling the product to exhibit a higher mechanical strength than those of the prior arts.
In the above conventional case, an attention has been mainly paid to the re-compaction step of the molded body, i.e., it has been intended to enhance the density thereof by the re-compaction step in order to obtain a product having a relatively high mechanical strength. However, the product obtained by the re-compaction step shows only a limited mechanical strength.
Consequently, in order to further enhance the mechanical strength of the product, it has been considered to be effective to increase a carbon content of the product, i.e., increase an amount of graphite added to a metal powder. However, in general, when the amount of graphite added increases, the molded body is deteriorated in elongation, and shows an increased hardness, thereby causing problems such as deteriorated deformability upon the re-compaction of the molded body and, therefore, difficulty in conducting the re-compaction step.
For example, in a pamphlet entitled xe2x80x9cThe Second Presentation of Developments in Powder Metallurgyxe2x80x9d, published by Japan Powder Metallurgy Association (Nov. 15, 1985), page 90, it has been described that a metallic powder-molded body having a carbon content of 0.05 to 0.5% exhibits an elongation of 10% at most, and a hardness of HRB 83. However, it is known from experience that a metallic powder-molded body having an elongation of not more than 10% and a hardness of more than HRB 60 is difficult to be re-compacted. For this reason, it has been required to obtain a metallic powder-molded body having a still higher elongation, a low hardness and an excellent deformability.
The present inventors have continuously made intense studies for producing various structural machine parts having a high mechanical strength due to the use of sintered metals. As a result, it has been recognized that when machine parts are manufactured by provisionally sintering a preform to form a metallic powder-molded body, re-compacting the molded body and subjecting the re-compacted body to substantial sintering, the metallic powder-molded body bears important factors determinate to qualities of the obtained machine parts. Therefore, it is necessary to obtain a molded body having a predetermined graphite content, a large elongation, a low hardness and an excellent deformability. Based on the above recognition, the present inventors have conducted further researches.
As a result of the researches, it has been found that the properties of the metallic powder-molded body having a predetermined graphite content, especially elongation and hardness thereof which are important properties for facilitating the re-compaction, are influenced and determined by a density of the preform prior to the formation of the molded body, a structure of the molded body obtained by provisionally sintering the preform, and the configuration of carbon contained in the molded body.
The present invention has been made in view of the above-described conventional problems. An object of the present invention is to provide a metallic powder-molded body having an excellent deformability, a re-compacted body of the molded body, a sintered body produced from the re-compacted body, and processes for the production thereof.
According to the invention as recited in at least certain claims, there is provided a metallic powder-molded body produced by a process comprising the steps of:
compacting a metallic powder mixture obtained by blending graphite with an iron-based metal powder to form a preform having a density of not less than 7.3g/cm3; and
provisionally sintering said preform at a temperature of 700-1000xc2x0 C.,
said metallic powder-molded body having a structure in which the graphite remains along a grain boundary of the metal powder.
In the invention as recited in at least certain claims, the amount of the graphite blended with the metal powder is 0.3% by weight or more.
According to the invention as recited in at least certain claims, there is provided a re-compacted body produced by re-compacting the metallic powder-molded body as claimed in at least certain claims.
According to the invention as recited in claim 4, there is provided a process for producing a re-compacted body, comprising:
a preliminary molding step of compacting a metallic powder mixture obtained by blending graphite with an iron-based metal powder to form a preform having a density of not less than 7.3 g/cm3;
a provisional sintering step of provisionally sintering said preform at a temperature of 700-1000xc2x0 C. to form a metallic powder-molded body having a structure in which the graphite remains along a grain boundary of the metal powder; and
a re-compaction step of re-compacting said metallic powder-molded body.
According to the invention as recited in at least certain claims, said preliminary molding step further comprises the step of pressing the metallic powder mixture filled in a mold cavity of a forming die, by upper and lower punches,
said mold cavity being formed with a greater-diameter portion into which the upper punch is inserted, a smaller-diameter portion into which the lower punch is inserted, and a tapered portion connecting the greater-diameter and smaller-diameter portions with each other, and either one or both of so the upper and lower punches having a notch at an outer circumferential periphery of an end surface thereof facing the mold cavity to increase a volume of the mold cavity.
According to the invention as recited in at least certain claims, in the process as claimed in at least certain claims, the amount of the graphite blended with the metal powder is 0.3% by weight or more.
According to the invention as recited in at least certain claims, there is provided a sintered body produced by a process comprising the steps of:
compacting a metallic powder mixture obtained by blending graphite with an iron-based metal powder to form a preform having a density of not less than 7.3 g/cm3;
provisionally sintering the preform at a temperature of 700-1000xc2x0 C. to form a metallic powder-molded body having a structure in which the graphite remains along a grain boundary of the metal powder;
re-compacting the metallic powder-molded body to form a re-compacted body; and
re-sintering the re-compacted body at a predetermined temperature,
said sintered body having a structure in which the graphite particle is diffused or remains in the metal powder and along a grain boundary thereof at a predetermined rate.
According to the invention as recited in at least certain claims, in the sintered body as claimed in at least certain claims, the amount of the graphite blended with the metal powder is 0.3% by weight or more.
According to the invention as recited in at least certain claims, there is provided a process for producing a sintered body, comprising:
a preliminary molding step of compacting a metallic powder mixture obtained by blending graphite with an iron-based metal powder to form a preform having a density of not less than 7.3 g/cm3;
a provisional sintering step of provisionally sintering the preform at a temperature of 700-1000xc2x0 C. to form a metallic powder-molded body having a structure in which the graphite remains along a grain boundary of the metal powder;
a re-compaction step of re-compacting the metallic powder-molded body to form a re-compacted body; and
a re-sintering step of re-sintering the re-compacted body.
According to the invention as recited in at least certain claims, in the process as claimed in at least certain claims, said preliminary molding step further comprises the step of pressing the metallic powder mixture filled in a mold cavity of a forming die, by upper and lower punches,
said mold cavity being formed with a greater-diameter portion into which the upper punch is inserted, a smaller-diameter portion into which the lower punch is inserted, and a tapered portion connecting the greater-diameter and smaller-diameter portions with each other, and either one or both of the upper and lower punches having a notch at an outer circumferential periphery of an end surface thereof facing the mold cavity to increase a volume of the mold cavity.
According to the invention as recited in at least certain claims, in the process as claimed in at least certain claims, the amount of the graphite blended with the metal powder is 0.3% by weight or more.
According to the invention as recited in at least certain claims, there is provided a sintered body produced by a process comprising the steps of:
compacting a metallic powder mixture obtained by blending graphite with an iron-based metal powder to form a preform having a density of not less than 7.3 g/cm3;
provisionally sintering the preform at a temperature of 700-1000xc2x0 C. to form a metallic powder-molded body having a structure in which the graphite remains along a grain boundary of the metal powder;
re-compacting the metallic powder-molded body to form a re-compacted body;
re-sintering the re-compacted body at a predetermined temperature to form a sintered body having a structure in which the graphite is diffused or remains in the metal powder and along a grain boundary thereof at a predetermined rate; and
heat-treating the sintered body.
According to the invention as recited in at least certain claims, in the sintered body as claimed in at least certain claims, the amount of the graphite blended with the metal powder is 0.3% by weight or more.
According to the invention as recited in at least certain claims, there is provided a process for producing a sintered body, comprising:
a preliminary molding step of compacting a metallic powder mixture obtained by blending graphite with an iron-based metal powder to form a preform having a density of not less than 7.3 g/cm3;
a provisional sintering step of provisionally sintering the preform at a temperature of 700-1000xc2x0 C. to form a metallic powder-molded body having a structure in which the graphite particle remains along a grain boundary of the metal powder;
a re-compaction step of re-compacting the metallic powder-molded body to form a re-compacted body;
a re-sintering step of re-sintering the re-compacted body to form a sintered body; and
a heat treatment step of heat-treating the sintered body.
According to the invention as recited in at least certain claims, in the process as claimed in at least certain claims, said preliminary molding step further comprises the step of pressing the metallic powder mixture filled in a mold cavity of a forming die, by upper and lower punches,
said mold cavity being formed with a greater-diameter portion into which the upper punch is inserted, a smaller-diameter portion into which the lower punch is inserted, and a tapered portion connecting the greater-diameter and smaller-diameter portions with each other, and either one or both of the upper and lower punches having a notch at an outer circumferential periphery of an end surface thereof facing the mold cavity to increase a volume of the mold cavity.
According to the invention as recited in at least certain claims, in the process as claimed in at least certain claims, the amount of the graphite blended with the metal powder is 0.3% by weight or more.
According to the invention as recited in at least certain claims, the metallic powder mixture of the metallic powder-molded body as claimed in at least certain claims, is an iron-based alloy steel powder containing at least one alloy element selected from the group consisting of molybdenum (Mo), nickel (Ni), manganese (Mn), copper (Cu), chromium (Cr), tungsten (W), vanadium (V), cobalt (Co) and the like, which element is capable of forming a solid solution with a base material of the metal powder to enhance mechanical properties such as strength and hardenability, or capable of forming a precipitate such as carbide to enhance mechanical properties such as strength and hardness,
said metallic powder-molded body, when being provisionally sintered, having a structure in which the graphite remains along a grain boundary of the metal powder and which contains substantially no precipitate such as carbides of iron or the alloy elements.
According to the invention as recited in at least certain claims, the metallic powder mixture of the metallic powder-molded body as claimed in at least certain claims, is obtained by diffusing and depositing a powder containing as a main component, an alloy element selected from the group consisting of molybdenum (Mo), nickel (Ni), manganese (Mn), copper (Cu), chromium (Cr), tungsten (W), vanadium (V), cobalt (Co) and the like, which element is capable of forming a solid solution with a base material of the metal powder to enhance mechanical properties such as strength and hardenability, or capable of forming a precipitate such as carbide to enhance mechanical properties such as strength and hardness, onto said iron-based metal powder,
said metallic powder-molded body, when being provisionally sintered, having a structure in which the graphite remains along a grain boundary of the metal powder and which contains substantially no precipitate such as carbides of iron or the alloy element.
According to the invention as recited in at least certain claims, the metallic powder mixture of the metallic powder-molded body as claimed in at least certain claims, is obtained by blending a powder containing as a main component, an alloy element selected from the group consisting of molybdenum (Mo), nickel (Ni), manganese (Mn), copper (Cu), chromium (Cr), tungsten (W), vanadium (V), cobalt (Co) and the like, which element is capable of forming a solid solution with a base material of the metal powder to enhance mechanical properties such as strength and hardenability, or capable of forming a precipitate such as carbide to enhance mechanical properties such as strength and hardness, with the iron-based metal powder,
said metallic powder-molded body, when being provisionally sintered, having a structure in which the graphite remains along a grain boundary of the metal powder and which contains substantially no precipitate such as carbides of iron or the alloy element.
According to the invention as recited in at least certain claims, in the metallic powder-molded body as claimed in at least certain claims, the amount of the graphite blended with the metal powder is 0.1% by weight or more.
According to the invention as recited in at least certain claims, there is provided a re-compacted body produced by re-compacting the metallic powder-molded body as claimed in at least certain claims, wherein the re-compacted body has a dense structure containing substantially no voids.
According to the invention as recited in at least certain claims, in there-compacted body as claimed in at least certain claims, the amount of the graphite blended with the metal powder is 0.1% by weight or more.
According to the invention as recited in at least certain claims, there is provided a process for producing a re-compacted body, comprising:
a preliminary molding step of compacting the metallic powder mixture as claimed in at least certain claims to form a preform having a density of not less than 7.3 g/cm3;
a provisional sintering step of provisionally sintering the preform at a temperature of 700-1000xc2x0 C. to form a metallic powder-molded body having a structure in which the graphite remains along a grain boundary of the metal powder; and
a re-compaction step of re-compacting the metallic powder-molded body.
According to the invention as recited in at least certain claims, there is provided a sintered body obtained by re-sintering the re-compacted body as claimed in at least certain claims at a predetermined temperature, wherein the sintered body has a graphite-diffused structure and a graphite-remaining structure at a predetermined ratio determined depending on the predetermined re-sintering temperature.
According to the invention as recited in at least certain claims, there is provided a process for producing a sintered body, comprising:
a preliminary molding step of compacting the metallic powder mixture claimed in at least certain claims to form a preform having a density of not less than 7.3 g/cm3;
a provisional sintering step of provisionally sintering the preform at a temperature of 700-1000xc2x0 C. to form a metallic powder-molded body having a structure in which the graphite remains along a grain boundary of the metal powder;
a re-compaction step of re-compacting the metallic powder-molded body to form a re-compacted body; and
a re-sintering step of re-sintering the re-compacted body.
According to the invention as recited in at least certain claims, there is provided a sintered body produced by heat-treating the sintered body as claimed in at least certain claims, wherein the sintered body heat-treated has a hardened structure.
According to the invention as recited in at least certain claims, there is provided a process for producing a sintered body, comprising:
a preliminary molding step of compacting the metallic powder mixture as claimed in at least certain claims to form a preform having a density of not less than 7.3 g/cm3;
a provisional sintering step of provisionally sintering the preform at a temperature of 700-1000xc2x0 C. to form a metallic powder-molded body having a structure in which the graphite remains along a grain boundary of the metal powder;
a re-compaction step of re-compacting the metallic powder-molded body to form a re-compacted body; and
a re-sintering step of re-sintering the re-compacted body to form a sintered body; and
a heat treatment step of heat-treating the sintered body.
According to the invention as recited in at least certain claims, in the sintered body claimed in at least certain claims, the amount of the graphite blended with the metal powder is 0.1% by weight or more.
According to the invention as recited in at least certain claims, there is provided a re-compacted body produced by a process comprising the steps of:
forming a preform using a device comprising a forming die having a mold cavity to be filled with the metallic powder mixture, and upper and lower punches inserted into the forming die to press the metallic powder mixture, said mold cavity being formed with a greater-diameter portion into which the upper punch is inserted, a smaller-diameter portion into which the lower punch is inserted, and a tapered portion connecting the greater-diameter and smaller-diameter portions with each other, and either one or both of the upper and lower punches having a notch at an end surface thereof facing the mold cavity to increase a volume of the mold cavity:
provisionally sintering the preform at a temperature of 700-1000xc2x0 C. to form the metallic powder-molded body as claimed in at least certain claims; and
re-compacting the metallic powder-molded body to form a re-compacted body.
According to the invention as recited in at least certain claims, there is provided a process for producing a re-compacted body, comprising the steps of:
forming a preform using a device comprising a forming die having a mold cavity to be filled with the metallic powder mixture, and upper and lower punches inserted into the forming die to press the metallic powder mixture, said mold cavity being formed with a greater-diameter portion into which the upper punch is inserted, a smaller-diameter portion into which the lower punch is inserted, and a tapered portion connecting the greater-diameter and smaller-diameter portions with each other, and either one or both of the upper and lower punches having a notch at an end surface thereof facing the mold cavity to increase a volume of the mold cavity;
provisionally sintering the preform at a temperature of 700-1000xc2x0 C. to form the metallic powder-molded body as claimed in at least certain claims; and
re-compacting the metallic powder-molded body to form a re-compacted body.
According to the invention as recited in at least certain claims, in the re-compacted body as claimed in at least certain claims, the amount of the graphite blended with the metal powder is 0.1% by weight or more.
According to the invention as recited in at least certain claims, there is provided a sintered body produced by a process comprising the steps of:
forming a preform using a device comprising a forming die having a mold cavity to be filled with the metallic powder mixture, and upper and lower punches inserted into the forming die to press the metallic powder mixture, said mold cavity being formed with a greater-diameter portion into which the upper punch is inserted, a smaller-diameter portion into which the lower punch is inserted, and a tapered portion connecting the greater-diameter and smaller-diameter portions with each other, and either one or both of the upper and lower punches having a notch at an end surface thereof facing the mold cavity to increase a volume of the mold cavity;
provisionally sintering the preform at a temperature of 700-1000xc2x0 C. to form the metallic powder-molded body as claimed in at least certain claims;
re-compacting the metallic powder-molded body to form a re-compacted body; and
re-sintering the re-compacted body to form the sintered body.
According to the invention as recited in at least certain claims, there is provided a process for producing a sintered body, comprising the steps of:
forming a preform using a device comprising a forming die having a mold cavity to be filled with the metallic powder mixture, and upper and lower punches inserted into the forming die to press the metallic powder mixture, said mold cavity being formed with a greater-diameter portion into which the upper punch is inserted, a smaller-diameter portion into which the lower punch is inserted, and a tapered portion connecting the greater-diameter and smaller-diameter portions with each other, and either one or both of the upper and lower punches having a notch at an end surface thereof facing the mold cavity to increase a volume of the mold cavity;
provisionally sintering the preform at a temperature of 700-1000xc2x0 C. to form the metallic powder-molded body as claimed in at least certain claims;
re-compacting the metallic powder-molded body to form a re-compacted body; and
re-sintering the re-compacted body to form the sintered body.
According to the invention as recited in at least certain claims, in the sintered body as claimed in at least certain claims, the amount of the graphite blended with the metal powder is 0.1% by weight or more.
According to the invention as recited in at least certain claims, there is provided a sintered body produced by conducting the re-sintering as claimed in at least certain claims, wherein the re-sintering temperature is within a range of 700-1300xc2x0 C.
In the invention as recited in at least certain claims, the re-compacted body according to the present invention is produced by re-compacting a metallic powder-molded body (hereinafter referred to merely as xe2x80x9cmolded bodyxe2x80x9d). The molded body is produced by provisionally sintering a preform obtained by compacting a metallic powder mixture, at a temperature of 700-1000xc2x0 C.
The preform has a density of not less than 7.3 g/cm3. By controlling the density of the preform to not less than 7.3 g/cm3, the molded body obtained by provisionally sintering the preform can exhibit a large elongation and a low hardness.
The molded body obtained by provisionally sintering the preform having a density of not less than 7.3 g/cm3, has a structure in which the graphite remains along a grain boundary of the metal powder. This indicates that almost no carbon is diffused into an interior of crystals of the metal powder, or at least there is not caused such a condition that a whole amount of graphite in diffused into crystal grains to form a solid solution therewith or produce a carbide therein. More specifically, the metal powder shows a ferrite structure as a whole, or a structure in which pearlite is precipitated in the vicinity of graphite. For this reason, the above molded body can exhibit a large elongation, a low hardness and an excellent deformability.
In addition, in the preform having a density of not less than 7.3 g/cm3, voids between the metal powder particles are not continuous but isolated, thereby obtaining a molded body showing a large elongation after the provisional sintering. That is, when the voids between the metal powder particles are continuous, an atmospheric gas within a furnace is penetrated into an interior of the preform upon the provisional sintering, and a gas generated from graphite contained thereinside is diffused around so as to promote carburization of the provisional sintered preform. However, since the voids of the preform used in the present invention are isolated from each other, the above problems can be effectively prevented, thereby obtaining the molded body having a large elongation. Thus, since the preform is substantially free from diffusion of carbon upon the provisional sintering by controlling the density of the preform to not less than 7.3 g/cm3, the elongation of the obtained molded body is rarely influenced by the content of graphite. Further, it is indicated that since the preform is substantially free from the diffusion of carbon, the molded body obtained by provisionally sintering the preform shows a reduced hardness.
Also, upon the provisional sintering, the sintering due to surface-diffusion or melting extensively occurs at contact surfaces between the metal powder particles, so that the obtained molded body can exhibit a large elongation.
Thus, in accordance with the invention as recited in at least certain claims, it is possible to obtain a re-compacted body of the molded body which is suitable for the manufacture of machine parts having a high mechanical strength due to the use of sintered metals, and exhibits an excellent deformability.
In the invention as recited in at least certain claims, the metallic powder mixture is produced by blending not less than 0.3% by weight of graphite with an iron-based metal powder. By controlling the amount of graphite blended with the metal powder to not less than 0.3% by weight, the metallic powder mixture capable of producing high-carbon steel can be obtained.
In the invention as recited in at least certain claims, the re-compacted body according to the present invention, is produced by re-compacting the molded body. The re-compaction can enhance the mechanical strength of the molded body. In particular, when the molded body having a graphite content of not less than 0.3% by weight is re-compacted, the obtained re-compacted body can have the substantially same mechanical strength as those of cast/forging materials.
In the invention as recited in at least certain claims, the preform is produced at the preliminary molding step, and the molded body is produced by provisionally sintering the preform at the provisional sintering step. The re-compacted body is produced by re-compacting the molded body at the re-compaction step.
The preform has a density of not less than 7.3 g/cm3. By controlling the density of the preform to not less than 7.3 g/cm3, the molded body obtained by provisionally sintering the preform at the provisional sintering step can exhibit a large elongation and a low hardness.
The molded body obtained by provisionally sintering the preform having a density of not less than 7.3 g/cm3 at the provisional sintering step, has a structure in which the graphite remains along a grain boundary of the metal powder. This indicates that almost no carbon is diffused into an interior of crystals of the metal powder, or at least, there is not caused such a condition that a whole amount of graphite is diffused into crystal grains to form a solid solution therewith or produce a carbide therein.
Specifically, the metal powder shows a ferrite structure as a whole, or a structure in which pearlite is precipitated in the vicinity of graphite. For this reason, the above molded body can exhibit a large elongation, a low hardness and an excellent deformability.
In addition, in the preform having a density of not less than 7.3 g/cm3, voids between the metal powder particles are not continuous but isolated, thereby obtaining a molded body showing a large elongation after the provisional sintering step. That is, when the voids between the metal powder particles are continuous, an atmospheric gas within a furnace is penetrated into an interior of the preform upon the provisional sintering, and a gas generated from graphite contained thereinside is diffused around so as to promote carburization of the provisionally sintered preform. However, since the voids of the preform used in the present invention are isolated from each other, the above problems can be effectively prevented, thereby obtaining the molded body having a large elongation. Thus, since the preform is substantially free from diffusion of carbon upon the provisional sintering by controlling the density of the preform to not less than 7.3 g/cm3, the elongation of the obtained molded body is rarely influenced by the graphite content. Further, it is indicated that since the preform is substantially free from the diffusion of carbon, the molded body obtained by provisionally sintering the preform shows a reduced hardness.
Also, upon the provisional sintering step, the sintering due to surface-diffusion or melting extensively occurs at contact surfaces between the metal powder particles, so that the obtained molded body can exhibit a large elongation.
In the invention as recited in at least certain claims, the provisional sintering temperature used at the provisional sintering step is within the range of 700-1000xc2x0 C., so that it is possible to obtain the molded body having a structure in which the graphite remains along a grain boundary of the metal powder which can exhibit an excellent deformability, i.e., an elongation of not less than 10% and a hardness of not more than HRB 60.
In the invention as recited in at least certain claims, the preliminary molding step of forming the preform is conducted by pressing the metallic powder mixture filled in a mold cavity of a forming die, by upper and lower punches. In this case, the density of the preform is as high as not less than 7.3 g/cm3 as a whole, so that the friction between the compact and the forming die increases. However, since a notch is formed at either one or both of the upper and lower punches, the density of the preform is locally reduced, so that the friction between the compact and the forming die can be reduced. For this reason, the preform is readily released from the forming die by the synergistic effect with the tapered portion formed within the mold cavity, thereby obtaining the preform having a density of not less than 7.3 g/cm3.
The re-compaction step is conducted preferably at ordinary temperature. In this case, the molded body can be readily re-compacted due to an excellent deformability thereof.
Thus, the re-compaction step can be performed by applying a small molding load to the molded body, thereby obtaining a re-compacted body with a high dimensional accuracy. The re-compacted body has such a structure in which metal particles of the molded body are largely deformed into a flat shape. However, since the molded body itself has the structure in which the graphite remains along a grain boundary of the metal powder, the obtained re-compacted body is excellent in machinability and lubricating ability.
Therefore, according to the invention as recited in at least certain claims, there is provided a process for the production of a re-compacted body having an excellent deformability, which is suitable for the manufacture of machine parts having a high mechanical strength due to the use of sintered metals.
In the invention as recited in at least certain claims, the metallic powder mixture compacted at the preliminary molding step as recited in at least certain claims, is produced by blending graphite with an iron-based metal powder. Among others, by controlling the amount of graphite blended with the metal powder to not less than 0.3% by weight, the sintered body obtained by re-compacting and re-sintering the molded body can show substantially the same mechanical strength as those of cast/forging materials.
In the invention as recited in at least certain claims, the sintered body is obtained by re-sintering the re-compacted body at a predetermined temperature. The re-compacted body is produced by re-compacting the molded body which is produced by provisionally sintering the preform obtained by compacting the metallic powder mixture, at a temperature of 700-1000xc2x0 C.
The preform has a density of not less than 7.3 g/cm3. By controlling the density of the preform to not less than 7.3 g/cm3, the molded body obtained by provisionally sintering the preform can exhibit a large elongation and a low hardness.
The molded body obtained by provisionally sintering the preform having a density of not less than 7.3 g/cm3, has a structure in which the graphite remains along a grain boundary of the metal powder. This indicates that almost no carbon is diffused into an interior of crystals of the metal powder, or at least there is not caused such a condition that a whole amount of graphite is diffused into crystal grains of the metal powder to form a solid solution therewith or produce a carbide therein. Specifically, the metal powder shows a ferrite structure as a whole, or a structure in which pearlite is precipitated in the vicinity of graphite. For this reason, the above molded body can exhibit a large elongation, a low hardness and an excellent deformability.
In addition, in the preform having a density of not less than 7.3 g/cm3, voids between the metal powder particles are not continuous but isolated, thereby obtaining a molded body showing a large elongation after the provisional sintering at the provisional sintering step. That is, when the voids between the metal powder particles are continuous, an atmospheric gas within a furnace is penetrated into an interior of the preform upon the provisional sintering, and a gas generated from graphite contained thereinside is diffused around so as to promote carburization of the provisional sintered preform. However, since the voids of the preform used in the present invention are isolated from each other, the above problems can be effectively prevented, thereby obtaining the molded body having a large elongation. Thus, since the preform is substantially free from diffusion of carbon upon the provisional sintering by controlling the density of the preform to not less than 7.3 g/cm3, the elongation of the obtained molded body is rarely influenced by the content of graphite. Further, it is indicated that since the preform is substantially free from the diffusion of carbon, the molded body obtained by provisionally sintering the preform shows a reduced hardness.
Also, upon the provisional sintering, the sintering due to surface-diffusion or melting extensively occurs at contact surfaces between the metal powder particles, so that the obtained molded body can exhibit a large elongation.
The re-compaction of the molded body obtained by provisionally sintering the preform is preferably conducted at ordinary temperature. In this case, owing to the excellent deformability, the molded body can be readily re-compacted by applying a small load thereto, thereby obtaining a re-compacted body having a high dimensional accuracy.
The re-compacted body is re-sintered to obtain a sintered body. The sintered body has a structure in which the graphite retained along a grain boundary of the metal powder is diffused into a ferrite base material (to form a solid solution or a carbide therewith), and a structure in which the graphite is diffused or remains in a ferrite or pearlite structure of the metal powder in a predetermined ratio. Here, the predetermined ratio includes no amount of the residual graphite.
The residual rate of the graphite varies depending upon the re-sintering temperature. The higher the re-sintering temperature is, the smaller the residual rate of the graphite becomes. By controlling the residual rate, the obtained sintered body can show desired mechanical properties such as mecahnical strength.
Therefore, according to the invention as recited in at least certain claims, it is possible to produce a sintered body by re-sintering a re-compacted body of the molded body having an excellent deformability, which is suitable for the manufacture of machine parts having a high mechanical strength due to the use of sintered metals.
In the invention as recited in at least certain claims, the metallic powder mixture is obtained by blending not less than 0.3% by weight of graphite with an iron-based metal powder. By controlling the amount of graphite blended with the metal powder to not less than 0.3% by weight, the sintered body obtained by re-compacting and re-sintering the molded body can show substantially the same mechanical strength as those of cast/forging materials.
In the invention as recited in at least certain claims, the preform is produced at the preliminary molding step, the molded body is produced by provisionally sintering the preform at the provisional sintering step, the re-compacted body is produced by re-compacting the molded body at the re-compaction step, the sintered body is produced by re-sintering the re-compacted body.
The preform formed at the preliminary molding step has a density of not less than 7.3 g/cm3. By controlling the density of the preform to not less than 7.3 g/cm3, the molded body obtained by provisionally sintering the preform at the provisional sintering step can exhibit a large elongation and a low hardness.
The molded body obtained by provisionally sintering the preform having a density of not less than 7.3 g/cm3, has a structure in which the graphite remains along a grain boundary of the metal powder. This indicates that almost no carbon is diffused into an interior of crystals of the metal powder, or at least there is not caused such a condition that a whole amount of graphite is diffused into crystal grains of the metal powder to form a solid solution therewith or produce a carbide therein. Specifically, the metal powder shows a ferrite structure as a whole, or a structure in which pearlite is precipitated in the vicinity of graphite. For this reason, the above molded body can exhibit a large elongation, a low hardness and an excellent deformability.
In addition, in the preform having a density of not less than 7.3 g/cm3, voids between the metal powder particles are not continuous but isolated, thereby obtaining a molded body showing a large elongation after the provisional sintering at the provisional sintering step. That is, when the voids between the metal powder particles are continuous, an atmospheric gas within a furnace is penetrated into an interior of the preform upon the provisional sintering, and a gas generated from graphite contained thereinside is diffused around so as to promote carburization of the provisional sintered preform. However, since the voids of the preform used in the present invention are isolated from each other, the above problems can be effectively prevented, thereby obtaining the molded body having a large elongation. Thus, since the preform is substantially free from diffusion of carbon upon the provisional sintering by controlling the density of the preform to not less than 7.3 g/cm3, the elongation of the obtained molded body is rarely influenced by the content of graphite. Further, it is indicated that since the preform is substantially free from the diffusion of carbon, the molded body obtained by provisionally sintering the preform shows a reduced hardness.
Also, at the provisional sintering step, the sintering due to surface-diffusion or melting extensively occurs at contact surfaces between the metal powder particles, so that the obtained molded body can exhibit a large elongation.
The provisional sintering temperature used at the provisional sintering step is selected within the range of 700-1000xc2x0 C., so that it is possible to obtain the molded body having a structure in which the graphite remains along a grain boundary of the metal powder, and exhibiting an excellent deformability, i.e., an elongation of not less than 10% and a hardness of not more than HRB 60.
The re-compaction step is preferably conducted at ordinary temperature. In this case, owing to the excellent deformability, the molded body can be readily re-compacted.
For this reason, the re-compacted body having a high dimensional accuracy can be obtained by applying a small load to the molded body.
The re-compacted body is re-sintered to obtain a sintered body. The sintered body has a structure in which the graphite retained along a grain boundary of the metal powder is diffused into a ferrite base material (to form a solid solution or a carbide therewith), and a structure in which the graphite is diffused or remains in a ferrite or pearlite structure of the metal powder in a predetermined ratio. Here, the predetermined ratio includes no amount of the residual graphite.
The residual rate of the graphite in the sintered body varies depending upon the re-sintering temperature. The higher the re-sintering temperature is, the smaller the residual rate of the graphite becomes. By controlling the residual rate, the obtained sintered body can show desired mechanical properties such as mechanical strength.
Therefore, according to the invention as recited in at least certain claims, it is possible to produce a sintered body by re-sintering the re-compacted body of the molded body having an excellent deformability, which is suitable for the manufacture of machine parts having a high mechanical strength due to the use of sintered metals.
In the invention as recited in at least certain claims, the preliminary molding step of forming the preform is conducted by pressing the metallic powder mixture filled in a mold cavity of a forming die, by upper and lower punches. In this case, the density of the obtained preform is as high as not less than 7.3 g/cm3 as a whole, so that the friction between the preform and the forming die increases. However, since a notch is formed at either one or both of the upper and lower punches, the density of the preform is locally reduced, so that the friction between the preform and the forming die can be lessened. For this reason, the preform is readily released from the forming die along with the synergistic effect of the tapered portion formed within the mold cavity, thereby obtaining the preform having a density of not less than 7.3 g/cm3.
In the invention as recited in at least certain claims, the metallic powder mixture is obtained by blending not less than 0.3% by weight of graphite with an iron-based metal powder. By controlling the amount of graphite blended with the metal powder to not less than 0.3% by weight, the sintered body obtained by re-compacting and re-sintering the molded body can show substantially the same mechanical strength as those of cast/forging materials.
In the invention as recited in at least certain claims, the sintered body is produced by heat-treating such a sintered body obtained by re-sintering the re-compacted body, at a predetermined temperature. The re-compacted body is produced by re-compacting the molded body. The molded body is produced by provisionally sintering the preform obtained by compacting the metallic powder mixture, at a predetermined temperature.
The preform has a density of not less than 7.3 g/cm3. By controlling the density of the preform to not less than 7.3 g/cm3, the molded body obtained by provisionally sintering the preform can exhibit a large elongation and a low hardness.
The molded body obtained by provisionally sintering the preform having a density of not less than 7.3 g/cm3, has a structure in which the graphite remains along a grain boundary of the metal powder. This indicates that almost no carbon is diffused into an interior of crystals of the metal powder, or at least there is not caused such a condition that a whole amount of graphite is diffused into crystal grains of the metal powder to form a solid solution therewith or produce a carbide therein. Specifically, the metal powder shows a ferrite structure as a whole, or a structure in which pearlite is precipitated in the vicinity of graphite. For this reason, the above molded body can exhibit a large elongation, a low hardness and an excellent deformability.
In addition, in the preform having a density of not less than 7.3 g/cm3, voids between the metal powder particles are not continuous but isolated, thereby obtaining a molded body showing a large elongation after the provisional sintering at the provisional sintering step. That is, when the voids between the metal powder particles are continuous, an atmospheric gas within a furnace is penetrated into an interior of the preform upon the provisional sintering, and a gas generated from graphite contained thereinside is diffused around so as to promote carburization of the provisionally sintered preform. However, since the voids of the preform used in the present invention are isolated from each other, the above problems can be effectively prevented, thereby obtaining the molded body having a large elongation. Thus, since the preform is substantially free from diffusion of carbon upon the provisional sintering by controlling the density of the preform to not less than 7.3 g/cm3, the elongation of the obtained molded body is rarely influenced by the content of graphite. Further, it is indicated that since the preform is substantially free from the diffusion of carbon, the molded body obtained by provisionally sintering the preform shows a reduced hardness.
Also, upon the provisional sintering, the sintering due to surface-diffusion or melting extensively occurs at contact surfaces between the metal powder particles, so that the obtained molded body can exhibit a large elongation.
The re-compaction of the molded body obtained by provisionally sintering the preform is preferably conducted at ordinary temperature. In this case, owing to the excellent deformability, the molded body can be readily re-compacted.
The re-compacted body is re-sintered to obtain a sintered body. The sintered body has a structure in which the graphite retained along a grain boundary of the metal powder is diffused into a ferrite base material (to form a solid solution or a carbide therewith), and a structure in which the graphite is diffused or remains in a ferrite or pearlite structure of the metal powder in a predetermined ratio. Here, the predetermined ratio includes no amount of the residual graphite.
The residual rate of the graphite in the sintered body varies depending upon the re-sintering temperature. The higher the re-sintering temperature is, the smaller the residual rate of the graphite becomes. By controlling the residual rate, the obtained sintered body can show desired mechanical properties such as mechanical strength.
The sintered body obtained by re-sintering the re-compacted body at a predetermined temperature is then heat-treated. The heat treatment may include various treatments such as induction quenching, carburizing and quenching, nitriding and the combination thereof. The sintered body obtained by re-sintering the re-compacted body at a predetermined temperature has a less amount of voids and a high density owing to the re-compaction, so that the degree of diffusion of carbon due to the heat treatment is gradually lessened inwardly from the surface of the sintered body. For this reason, the heat-treated sintered body shows an increased hardness in the vicinity of the surface thereof, and a toughness at an inside thereof, thereby allowing the sintered body to have an excellent mechanical properties as a whole.
Therefore, according to the invention as recited in at least certain claims, the sintered body which is suitable for the manufacture of machine parts having a high mechanical strength due to the use of sintered metals, can be obtained by heat-treating the sintered body obtained by re-sintering the re-compacted body of the molded body having an excellent deformability.
In the invention as recited in at least certain claims, the metallic powder mixture is obtained by blending not less than 0.3% by weight of graphite with an iron-based metal powder. By controlling the amount of graphite blended with the metal powder to not less than 0.3% by weight, the sintered body obtained by re-compacting and re-sintering the molded body can show substantially the same mechanical strength as those of cast/forging materials.
In the invention as recited in at least certain claims, by controlling the density of the preform to not less than 7.3 g/cm3, the molded body obtained by provisionally sintering the. preform at the provisional sintering step can exhibit a large elongation and a low hardness.
The molded body obtained by provisionally sintering the preform having a density of not less than 7.3 g/cm3 at the provisional sintering step, has a structure in which the graphite remains along a grain boundary of the metal powder. This indicates that almost no carbon is diffused into an interior of crystals of the metal powder, or at least, there is not caused such a condition that a whole amount of graphite is diffused into crystal grains of the metal powder to form a solid solution therewith or produce a carbide therein. Specifically, the metal powder shows a ferrite structure as a whole, or a structure in which pearlite is precipitated in the vicinity of graphite. For this reason, the above molded body can exhibit a large elongation, a low hardness and an excellent deforambility.
In addition, in the preform having a density of not less than 7.3 g/cm3, voids between the metal powder particles are not continuous but isolated, thereby obtaining a molded body showing a large elongation after the provisional sintering at the provisional sintering step. That is, if the voids between the metal powder particles are continuous, an atmospheric gas within a furnace is penetrated into an interior of the preform upon the provisional sintering, and a gas generated from graphite contained thereinside is diffused around so as to promote carburization of the provisionally sintered preform. However, since the voids of the preform used in the present invention are isolated from each other, the above problems can be effectively prevented, thereby obtaining the molded body having a large elongation. Thus, since the preform is substantially free from diffusion of carbon upon the provisional sintering by controlling the density of the preform to not less than 7.3 g/cm3, the elongation of the obtained molded body is rarely influenced by the content of graphite. Further, it is indicated that since the preform is substantially free from the diffusion of carbon, the molded body obtained by provisionally sintering the preform shows a reduced hardness.
Also, upon the provisional sintering at the provisional sintering step, the sintering due to surface-diffusion or melting extensively occurs at contact surfaces between the metal powder particles, so that the obtained molded body can exhibit a large elongation.
The provisional sintering temperature used at the provisional sintering step is selected within the range of 700-1000xc2x0 C., so that it is possible to obtain the molded body having a structure in which the graphite remains along a grain boundary of the metal powder, and exhibiting an excellent deformability, i.e., an elongation of not less than 10% and a hardness of not more than HRB 60.
The re-compaction step is preferably conducted at ordinary temperature. In this case, owing to the excellent deformability, the molded body can be readily re-compacted.
For this reason, the re-compacted body having a high dimensional accuracy can be obtained by applying a small load to the molded body.
At the re-sintering step, the re-compacted body is re-sintered to obtain a sintered body. The sintered body has a structure in which the graphite retained along a grain boundary of the metal powder is diffused into a ferrite base material (to form a solid solution or a carbide therewith), and in which the graphite is diffused or remains in a ferrite or pearlite structure of the metal powder in a predetermined ratio. Here, the predetermined ratio includes no amount of the residual graphite.
The residual rate of the graphite in the sintered body varies depending upon the re-sintering temperature. The higher the re-sintering temperature is, the smaller the residual rate of the graphite becomes. By controlling the residual rate, the obtained sintered body can show desired mechanical properties such as mechanical strength.
The sintered body obtained by re-sintering the re-compacted body at a predetermined temperature is then heat-treated. The heat treatment may include various treatments such as induction quenching, carburizing and quenching, nitriding and the combination thereof. The sintered body obtained by re-sintering the re-compacted body at a predetermined temperature has a less amount of voids and a high density owing to the re-compaction, so that the degree of diffusion of carbon due to the heat treatment is gradually lessened inwardly from the surface of the sintered body. For this reason, the heat-treated sintered body shows an increased hardness in the vicinity of the surface thereof, and a toughness at an inside thereof, thereby allowing the sintered body to have excellent mechanical properties as a whole.
In the invention as recited in at least certain claims, the metallic powder mixture filled in a mold cavity of a forming die, is pressed by upper and lower punches. In this case, the density of the obtained preform is as high as not less than 7.3 g/cm3, so that the friction between the preform and the forming die increases. However, since a notch is formed at either one or both of the upper and lower punches, the density of the preform is locally reduced, so that the friction between the preform and the forming die can be lessened. For this reason, the preform is readily released from the forming die along with the synergistic effect of the tapered portion formed within the mold cavity, thereby obtaining the preform having a density of not less than 7.3 g/cm3.
Further, in the invention as recited in at least certain claims, the metallic powder mixture compacted at the preliminary molding step as recited in at least certain claims, is obtained by blending not less than 0.3% by weight of graphite with an iron-based metal powder. By controlling the amount of graphite blended with the metal powder to not less than 0.3% by weight, the sintered body obtained by re-compacting and re-sintering the molded body can show substantially the same mechanical strength as those of cast/forging materials.
In the inventions as recited in at least certain claims, the preform obtained by the compaction of the metallic powder mixture has a density of not less than 7.3 g/cm3. Therefore, the molded body obtained by provisionally sintering the preform contains the graphite that surely remains along a grain boundary of the metal powder. As a result, the molded body can show a low hardness, a large elongation, a high lubricating ability along the grain boundary of the metal powder, and a high moldability as a whole.
That is, in the preform compacted into a high density of not less than 7.3 g/cm3, voids between the metal powder particles are not continuous but isolated, so that it becomes difficult to penetrate an atmospheric gas within a furnace into the preform upon the provisional sintering, and diffuse a gas generated from graphite contained thereinside to the surrounding. This considerably contributes to inhibiting the diffusion of carbon (to allow the residual graphite). For this reason, the obtained molded body has a structure in which the graphite remains along a grain boundary of the metal powder and almost no precipitates such as carbides of iron or alloy elements are formed.
Specifically, the mold preform as recited in at least certain claims has a ferrite structure, an austenite structure or such a structure in which a slight amount of pearlite or bainite is precipitated in the vicinity of graphite. Whereas, the molded body as recited in at least certain claims has a ferrite structure, an austenite structure, a structure in which at least one undiffused alloy component such as nickel (Ni) is co-present, or a structure in which a slight amount of pearlite or bainite is precipitated in the vicinity of graphite. Therefore, the molded body before subjecting to the re-compaction, is rarely influenced by the diffusion of carbon. As a result, the molded body not only shows a low hardness and a large elongation, but also is further enhanced in moldability since the grain boundary of the metal powder is well lubricated by the residual graphite.
Also, upon the provisional sintering of the molded body, the sintering due to surface diffusion or melting is extensively caused at contact surfaces between the metal powder particles, thereby obtaining a molded body with a large elongation.
In the invention as recited in at least certain claims, the metallic powder mixture such as alloy steel powder contains not less than 0.1% by weight of graphite, so that when the preform is provisionally sintered or the obtained molded body is re-sintered, the decarburization of substantially a whole amount of carbon is prevented. Therefore, machine parts obtained by re-compacting and re-sintering the molded body can show a sufficiently enhanced mechanical strength.
In the invention as recited in at least certain claims, the re-compacted body obtained by subjecting the molded body to re-compaction such as cold forging, has a dense structure in which the graphite still remains along a grain boundary of the metal powder, but voids of the molded body are collapsed and almost entirely dissipated.
Also, since the molded body used therein is substantially free from diffusion of carbon, it is possible to re-compact the molded body into a desired shape by applying a small molding load (deformation resistance) thereto. Specifically, if a large amount of carbon is diffused in the molded body (like conventional molded bodies), the molded body shows not only a high hardness and a small elongation, but also a low sliding property between the metal particles, so that it becomes very difficult to re-compact the molded body. On the contrary, the molded body used in the present invention is substantially free from diffusion of carbon. Therefore, the molded body can show a low hardness and a large elongation and surely exhibits a good sliding property between the metal particles due to the graphite remaining along a grain boundary thereof. As a result, it becomes possible to re-compact the molded body. Further, since the re-compaction of the molded body is conducted at ordinary temperature, production of scales or deteriorated dimensional accuracy of the re-compacted body due to transformation thereof can be prevented, thereby enabling the re-compacted body to be processed with an extremely high accuracy.
Further, the alloy components added to the metallic powder mixture serves for enhancing the degree of work-hardening upon the re-compaction. The plastic-worked body produced therefrom shows a higher hardness as compared to the case where no alloy component is added. However, since the grain boundary is well lubricated by the residual graphite, the molded body can be re-compacted with a small deformation resistance. In particular, in the molded body as recited in at least certain claims, the diffused alloy components are exposed to the near-surface portion of the metal powder, so that the diffusion of the alloy components is difficult to proceed towards an inside of the metal powder. As a result, it is possible to obtain a plastic-worked body which is work-hardened with a lower deformation resistance.
Accordingly, the obtained plastic-worked body is applicable to sliding parts requiring a high strength and a high accuracy.
In the invention as recited in at least certain claims, the metallic powder mixture compacted at the preliminary molding step as recited in at least certain claims, is produced by blending not less than 0.1% by weight of graphite with an iron-based metal powder. By controlling the amount of graphite blended with the metal powder to not less than 0.1% by weight, the sintered body obtained by re-compacting and re-sintering the molded body can be enhanced in mechanical strength.
Specifically, the metallic powder mixture used herein is obtained by blending not less than 0.1% by weight of graphite with an alloy steel powder. Therefore, when the preform is provisionally sintered or the obtained molded body is subsequently re-sintered, the decarburization of substantially a whole amount of carbon can be prevented. Accordingly, the machine parts obtained by re-compacting and re-sintering the molded body can show substantially the same mechanical strength as those of cast/forging materials.
In the invention as recited in at least certain claims, by controlling the density of the preform compacted at the preliminary molding step to not less than 7.3 g/cm3, the molded body obtained by provisionally sintering the preform at the provisional sintering step can exhibit a large elongation and a low hardness.
The molded body obtained by provisionally sintering the preform having a density of not less than 7.3 g/cm3 at the provisional sintering step, has a structure in which the graphite remains along a grain boundary of the metal powder. This indicates that almost no carbon is diffused into an interior of crystals of the metal powder, or at least, there is not caused such a condition that a whole amount of graphite is diffused into crystal grains of the metal powder to form a solid solution therewith or produce a carbide therein.
Specifically, the metal powder shows a ferrite structure as a whole, or a structure in which pearlite is precipitated in the vicinity of graphite. For this reason, the above molded body can exhibit a large elongation, a low hardness and an excellent deformability.
In addition, in the preform having a density of not less than 7.3 g/cm3, voids between the metal powder particles are not continuous but isolated from each other, thereby obtaining a molded body showing a large elongation after the provisional sintering at the provisional sintering step. That is, if the voids between the metal powder particles are continuous, an atmospheric gas within a furnace is penetrated into an interior of the preform upon the provisional sintering, and a gas generated from graphite contained thereinside is diffused around so as to promote carburization of the provisionally sintered preform. However, since the voids of the preform used in the present invention are isolated from each other, the above problems can be effectively prevented, thereby obtaining the molded body having a large elongation. Thus, since the preform is substantially free from diffusion of carbon upon the provisional sintering by controlling the density of the preform to not less than 7.3 g/cm3, the elongation of the obtained molded body is rarely influenced by the content of graphite. Further, it is indicated that since the preform is substantially free from the diffusion of carbon, the molded body obtained by provisionally sintering the preform shows a reduced hardness.
Also, upon the provisional sintering at the provisional sintering step, the sintering due to surface-diffusion or melting extensively occurs at contact surfaces between the metal powder particles, so that the obtained molded body can exhibit a large elongation.
Further, the provisional sintering temperature used at the provisional sintering step is selected within the range of 700 to 1,000xc2x0 C., so that it is possible to obtain the molded body having a structure in which the graphite remains along a grain boundary of the metal powder, and exhibiting an excellent deformability, i.e., an elongation of not less than 10% and a hardness of not more than HRB 60.
By re-compacting the molded body, it is possible to obtain the re-compacted body having a dense structure in which almost no voids are present.
Further, the re-compacted body obtained by subjecting the molded body to re-compaction such as cold forging, has a dense structure in which the graphite still remains along a grain boundary of the metal powder, but voids of the molded body are collapsed and almost entirely dissipated.
In the invention as recited in at least certain claims, when the re-compacted body is re-sintered, the sintering due to surface-diffusion or melting occurs at contact surfaces between the metal powder particles and, at the same time, the graphite retained along a grain boundary of the metal powder is diffused into a ferrite base material of the metal powder (to form a solid solution or a carbide therewith). The metal powder has a ferrite structure, a pearlite structure, an austenite structure or such a structure in which at least one undiffused alloy component such as nickel (Ni) coexists. When the residual graphite is present, there is obtained such a structure in which graphite is interspersed inside the metal powder.
Further, upon the re-sintering, the alloy elements capable of forming a solid solution with the base material can produce a more uniform solid solution therewith, and those capable of forming precipitates such as carbides can be formed into precipitates. Thus, the effect of enhancing mechanical properties by these alloy elements added, can be reflected on the macrostructure of the sintered body.
As a result, the obtained sintered body has a higher strength than that of the re-compacted body, and can exhibit a mechanical strength substantially identical to or higher than those of cast/forging materials which do not particularly require a hardened layer.
In addition, the thus obtained sintered body shows a re-crystallized structure having a crystal grain size of about 20 xcexcm or smaller due to the re-sintering after the re-compaction. This allows the sintered body to exhibit a high strength, a large elongation, a high impact value and a high fatigue strength.
In the invention as recited in at least certain claims, by controlling the density of the preform compacted at the preliminary molding step to not less than 7.3 g/cm3, the molded body obtained by provisionally sintering the preform at the provisional sintering step can exhibit a large elongation and a low hardness.
The molded body obtained by provisionally sintering the preform having a density of not less than 7.3 g/cm3 at the provisional sintering step, has a structure in which the graphite remains along a grain boundary of the metal powder. This indicates that almost no carbon is diffused into an interior of crystals of the metal powder, or at least, there is not caused such a condition that a whole amount of graphite is diffused into crystal grains of the metal powder to form a solid solution therewith or produce a carbide therein. Specifically, the metal powder shows a ferrite structure as a whole, or a structure in which pearlite is precipitated in the vicinity of graphite. For this reason, the above molded body can exhibit a large elongation, a low hardness and an excellent deformability.
In addition, in the preform having a density of not less than 7.3 g/cm3, voids between the metal powder particles are not continuous but isolated from each other, thereby obtaining a molded body showing a large elongation after the provisional sintering at the provisional sintering step. That is, if the voids between the metal powder particles are continuous, an atmospheric gas within a furnace is penetrated into an interior of the preform upon the provisional sintering, and a gas generated from graphite contained thereinside is diffused around so as to promote carburization of the provisionally sintered preform. However, since the voids of the preform used in the present invention are isolated from each other, the above problems can be effectively prevented, thereby obtaining the molded body having a large elongation. Thus, since the preform is substantially free from diffusion of carbon upon the provisional sintering by controlling the density of the preform to not less than 7.3 g/cm3, the elongation of the obtained molded body is rarely influenced by the content of graphite. Further, it is indicated that since the preform is substantially free from the diffusion of carbon, the molded body obtained by provisionally sintering the preform shows a reduced hardness.
Also, upon the provisional sintering step, the sintering due to surface-diffusion or melting extensively occurs at contact surfaces between the metal powder particles, so that the obtained molded body can exhibit a large elongation.
The provisional sintering temperature used at the provisional sintering step is selected without the range of 700-1000xc2x0 C., so that it is possible to obtain the molded body having a structure in which the graphite remains along a grain boundary of the metal powder, and exhibiting an excellent deformability, i.e., an elongation of not less than 10% and a hardness of not more than HRB 60.
The re-compaction step is preferably conducted at ordinary temperature. In this case, owing to the excellent deformability, the molded body can be readily re-compacted.
For this reason, the re-compacted body having a high dimensional accuracy can be obtained by applying a small load to the molded body.
The re-compacted body is re-sintered at the re-sintering step to obtain a sintered body. The sintered body has a structure in which the graphite retained along a grain boundary of the metal powder is diffused into a ferrite base material (to form a solid solution or a carbide therewith), and a structure in which the graphite is diffused or remains in a ferrite or pearlite structure of the metal powder in a predetermined ratio. Here, the predetermined ratio includes no amount of the residual graphite.
The residual rate of the graphite in the sintered body varies depending upon the re-sintering temperature. The higher the re-sintering temperature is, the smaller the residual rate of the graphite becomes. By controlling the residual rate, the obtained sintered body can show desired mechanical properties such as mechanical strength.
Therefore, according to the invention as recited in at least certain claims, there is provided a process for the production of a sintered body by re-sintering the re-compacted body of the molded body having an excellent deformability, which is suitable for the manufacture of machine parts having a high mechanical strength due to the use of sintered metals.
In the invention as recited in at least certain claims, when the sintered body is subjected to the heat treatment such as quenching, the graphite forms a super-saturated solid solution therewith, or is precipitated in the form of fine carbides or nitrides the latter of which produce a hardened layer. Therefore, in the obtained sintered body, the degree of diffusion of carbon caused by the heat treatment becomes lessened towards an inside thereof. The obtained sintered body thus shows a high hardness at the near-surface portion, while maintaining a good toughness thereinside.
In the invention as recited in at least certain claims, by controlling the density of the preform compacted at the preliminary molding step to not less than 7.3 g/cm3, the molded body obtained by provisionally sintering the preform at the provisional sintering step can exhibit a large elongation and a low hardness.
The molded body obtained by provisionally sintering the preform having a density of not less than 7.3 g/cm3 at the provisional sintering step, has a structure in which the graphite remains along a grain boundary of the metal powder. This indicates that almost no carbon is diffused into an interior of crystals of the metal powder, or at least, there is not caused such a condition that a whole amount of graphite is diffused into crystal grains of the metal powder to form a solid solution therewith or produce a carbide therein. Specifically, the metal powder shows a ferrite structure as a whole, or a structure in which pearlite is precipitated in the vicinity of graphite. For this reason, the above molded body can exhibit a large elongation, a low hardness and an excellent deformability.
In addition, in the preform having a density of not less than 7.3 g/cm3, voids between the metal powder particles are not continuous but isolated from each other, thereby obtaining a molded body showing a large elongation after the provisional sintering of the provisional sintering step. That is, if the voids between the metal powder particles are continuous, an atmospheric gas within a furnace is penetrated into an interior of the preform upon the provisional sintering, and a gas generated from graphite contained thereinside is diffused around so as to promote carburization of the provisionally sintered preform. However, since the voids of the preform used in the present invention are isolated from each other, the above problems can be effectively prevented, thereby obtaining the molded body having a large elongation. Thus, since the preform is substantially free from diffusion of carbon upon the provisional sintering by controlling the density of the preform to not less than 7.3 g/cm3, the elongation of the obtained molded body is rarely influenced by the content of graphite. Further, it is indicated that since the preform is substantially free from the diffusion of carbon, the molded body obtained by provisionally sintering the preform shows a reduced hardness.
Also, upon the provisional sintering at the provisional sintering step, the sintering due to surface-diffusion or melting extensively occurs at contact surfaces between the metal powder particles, so that the obtained molded body can exhibit a large elongation.
The provisional sintering temperature used at the provisional sintering step is selected within the range of 700-1000xc2x0 C., so that it is possible to obtain the molded body having a structure in which the graphite remains along a grain boundary of the metal powder, and exhibiting an excellent deformability, i.e., an elongation of not less than 10% and a hardness of not more than HRB 60.
The re-compaction step is preferably conducted at ordinary temperature. In this case, owing to the excellent deformability, the molded body can be readily re-compacted.
For this reason, the re-compacted body having a high dimensional accuracy can be obtained by applying a small load to the molded body.
The re-compacted body is re-sintered at the re-sintering step to obtain a sintered body. The sintered body has a structure in which the graphite retained along a grain boundary of the metal powder is diffused into a ferrite base material (to form a solid solution or a carbide therewith), and a structure in which the graphite is diffused or remains in a ferrite or pearlite structure of the metal powder in a predetermined ratio. Here, the predetermined ratio includes no amount of the residual graphite.
The residual rate of the graphite in the sintered body varies depending upon the re-sintering temperature. The higher the re-sintering temperature is, the smaller the residual rate of the graphite becomes. By controlling the residual rate, the obtained sintered body can show desired mechanical properties such as mechanical strength.
The sintered body obtained by re-sintering the re-compacted body at a predetermined temperature is then heat-treated. The heat treatment may include various treatments such as induction quenching, carburizing-quenching, nitriding and the combination thereof. The sintered body obtained by re-sintering the re-compacted body at a predetermined temperature has less amount of voids and a high density owing to the re-compaction, so that the degree of diffusion of carbon due to the heat treatment is lessened inwardly from the surface of the sintered body. For this reason, the heat-treated sintered body shows an increased hardness in the vicinity of the surface thereof, and a good toughness at an inside thereof, thereby allowing the sintered body to have excellent mechanical properties as a whole.
In the invention as recited in at least certain claims, by controlling the amount of graphite blended with the metal powder to not less than 0.1% by weight, the sintered body obtained by re-compacting and re-sintering the molded body can show substantially the same mechanical strength as those of cast/forging materials.
In the invention as recited in at least certain claims, it is required that the preform used for forming the molded body has a density as high as not less than 7.3 g/cm3. Therefore, it is considered that the friction upon releasing the preform from the forming die is considerably increased. However, in the apparatus used for the above operation, since a notch is formed at either one or both of the upper and lower punches thereof, the density of the preform is locally reduced, so that the friction generated upon the mold-releasing can be reduced. For this reason, the preform is readily released from the forming die along with the synergistic effect of the tapered portion formed within the mold cavity of the forming die, thereby obtaining the preform having a density of not less than 7.3 g/cm3.
The molded body obtained by provisionally sintering the preform surely has a high density to thereby contain a sufficient amount of the graphite remaining along the grain boundary of the metal powder and at the same time almost no carbon diffused into the metal particle. As a result, the subsequent re-compacting can be readily conducted. Accordingly, the re-compacted body has a dense structure containing substantially no voids and a high accuracy because the re-compaction at ordinary temperature is easily performed.
In the invention as recited in at least certain claims, there is provided a process for the production of a re-compacted body as recited in at least certain claims, by which the re-compacted body having the specific function and effects as recited in at least certain claims can be readily obtained.
In the invention as recited in at least certain claims, the re-compacted body as recited in at least certain claims is produced by blending not less than 0.1% by weight of graphite with the metal powder. By controlling the amount of graphite blended with the metal powder to not less than 0.1% by weight, the sintered body obtained by re-compacting and re-sintering the molded body can be enhanced in mechanical strength substantially as large as cast/forging materials.
In the invention as recited in at least certain claims, it is required that the preform used for forming the molded body has a density as high as not less than 7.3 g/cm3. Therefore, it is considered that the friction upon releasing the preform from the forming die is considerably increased. However, in the apparatus used for the above operation, since a notch is formed at either one or both of the upper and lower punches thereof, the density of the preform is locally reduced, so that the friction generated upon the mold-releasing can be reduced. For this reason, the preform is readily released from the forming die along with the synergistic effect of the tapered portion formed within the mold cavity of the forming die, thereby obtaining the preform having a density of not less than 7.3 g/cm3.
Also, the molded body obtained by provisionally sintering the preform surely has a high density to thereby contain a sufficient amount of the graphite remaining along the grain boundary of the metal powder and at the same time almost no carbon diffused into the metal particle. As a result, the subsequent re-compacting can be readily conducted. Accordingly, the re-compacted body has a dense structure containing substantially no voids and a high accuracy because the re-compaction at ordinary temperature is easily performed.
The re-compacted body is re-sintered to obtain a sintered body. The sintered body has a structure in which the graphite retained along a grain boundary of the metal powder is diffused into a ferrite base material (to form a solid solution or a carbide therewith), and a structure in which the graphite is diffused or remains in a ferrite or pearlite structure of the metal powder in a predetermined ratio. Here, the predetermined ratio includes no amount of the residual graphite.
The residual rate of the graphite in the sintered body varies depending upon the re-sintering temperature. The higher the re-sintering temperature is, the smaller the residual rate of the graphite becomes. By controlling the residual rate, the obtained sintered body can show desired mechanical properties such as mechanical strength. Accordingly, the sintered body can be obtained by re-sintering the re-compacted body of the molded body having an excellent deformability, which is suitable for the manufacture of machine parts having a high mechanical strength due to the use of sintered metals.
In the invention as recited in at least certain claims, there is provided a process for the production of a sintered body as recited in at least certain claims, by which the sintered body having the specific function and effects as recited in at least certain claims can be readily obtained.
In the invention as recited in at least certain claims, by controlling the amount of graphite blended with the metal powder to not less than 0.1% by weight, the sintered body obtained by re-compacting and re-sintering the molded body can be enhanced in mechanical strength substantially as large as cast/forging materials.
In the invention as recited in at least certain claims, the re-sintering temperature as recited in at least certain claims is selected within the range of 700-1300xc2x0 C. By controlling the re-sintering temperature to the range of 700-1300xc2x0 C., it is possible to obtain the sintered body having a structure which show a less diffusion of the graphite with the increased residual rate thereof, at a low range of the re-sintering temperature and obtain the sintered body having a structure which show a large diffusion of the graphite with the lowered residual rate thereof and exhibit the small re-growth of crystal with the maximum strength at a high range of the re-sintering temperature.